When the audio amplifier is powered on, this produces unwanted noise which is called a “pop” by those skilled in the art. This pop is considered to be a defect audible to the user of the device. It is unpleasant to hear, particularly when listening through headphones or earphones.
FIG. 1 shows the evolution over time of the output voltage Vout(t) of the audio amplifier at power-on and in the absence of an input signal to the amplifier. The pop arises from the generation of a voltage peak 1, which is called a glitch, and the presence of a non-zero voltage level 2 corresponding to the offset voltage of the amplifier.
A glitch is a momentary disturbance in the amplitude of the output signal, such as in the case of a temporary impedance mismatch. It is due primarily to the transient state which follows activation of the circuit. Its origin lies in the delay required for the various circuit components to reach the operating point.
In practice, it is not uncommon to observe glitch voltages of around +500 mV for about 2 μs, depending on the load driven by the amplifier. Such a glitch is audible to the user.
Techniques exist for decreasing the unwanted noise in the audio signal reproduced for the user, caused by the glitch which occurs when power is turned on. Such a technique is presented in the document U.S. Pat. No. 7,313,244.
However, the solution in said document is unsuitable for the applications considered here, because it is incompatible with the constraints related to integrating an analog audio amplifier on a silicon chip.